1. Field of the Invention
The invention relates to a filter with automatic backwashing of the type having at least one pair of filter elements that are essentially annular and flat, each having a screen, and that are stacked along an axis. More particularly, the invention relates to the arrangement of means for automatically backwashing said screens.
2. Description of Related Art
The French patent published under the number 2 779 359 describes a filter structure using annular flat filter elements, each having an “internal” face, an “external” face, and a screen. The screen is annular and extends between two concentric circular margins, respectively an inner margin and an outer margin. Radial ribs that are regularly distributed circumferentially and that extend facing one another on opposite sides of the screen between said concentric margins serve to form sectors on the internal face and on the external face. Such filter elements are designed to be assembled against each other in such a manner that their facing internal faces define a space that is compartmented radially by the contacting radial ribs of said internal faces. For each sector, passages are arranged in the inner and outer margins. By stacking a selected number of filter element pairs, a desired filter area is obtained for an intended application. Typically, the fluid for filtering penetrates into the stack via passages situated radially on the inside, passes through the screens, and, once purified, penetrates into the various above-defined spaces to exit via the passages defined radially on the outside. Naturally, the flow of fluid between the screens could be reversed or arranged in some other way.
In that known prior art device, backwashing is used to unclog the filter by making use of the clean fluid that is available under pressure at the outlet from the filter. For example, a distributor is mounted axially inside the stack and is driven in rotation. It has a longitudinal opening suitable for coming into communication with all of the passages defined in the inner margins along a given generator line of the annular stack. The distributor is in communication with an exhaust outlet for the fluid carrying the impurities resulting from backwashing, sector by sector. During rotation of the distributor, the sectors are isolated one after another and no longer receive fluid for cleaning. On the contrary, a fraction of the fluid that has just been cleaned penetrates as a backwash into the isolated sector (which extends over the full height of the stack) and serves to unclog the portions of the screen that are defined by the sectors.
The fluid conveying the impurities that result from the backwashing can be processed by a second filter stage or by a centrifuge system. Periodic purges serve to clear the system of these impurities.
The above-described filter structure requires a rotary distributor that extends axially over the entire length of the stack of filter elements. Its length therefore depends on the size of the filter, which is an obstacle to reducing the cost price of such a filter.
Document FR 2 902 024 proposes another type of rotary distributor by associating each filter element with segments of longitudinal channels adjacent to one of said inner or outer margins and communicating respectively with the passages therein.
The stack of filter elements thus defines a set of parallel distribution columns extending circumferentially side by side along directions that are parallel to the axis of said stack. A rotary backwash distributor is mounted to rotate at one end of the stack so that its opening is periodically and selectively put into communication with each of the distribution columns.
That solution gives satisfaction for filter elements of small size. Nevertheless, as is known, such a filter element includes radial and circumferential ribs molded onto a filter screen. The channel segment needs to be made during the same operation. When making filter elements of large size, it has been found that it is difficult to inject the material constituting these ribs and the associated channel segments.
The invention serves in particular to solve that problem.